Chassis variants:

The first 6 digits of the chassisnumber designate the exact
type of the car (the baumuster number), so if the chassisnr. of
your car would be W126023000000001, the first six digits are
126023, designating a 280SEL. Needless to say, if you own a SEC
and number 4 - 6 are not 043, 044, 045 or 046, the number and the
car are likely to have been build separately :)